Polymer-based composite materials are used in applications requiring performance at temperatures ranging from below ambient to 500.degree. F. Typical applications include: compressor blades, ducts, splitters and thrust-vectoring flaps for jet engines, missile fins, wing components, radar domes, and other aerospace structures. Polyimide resins have been employed in such applications due to their high temperature and thermal stability properties.
Polyimide resins are generally produced either by condensation polymerization directly or by addition polymerization followed by a condensation rearrangement reaction to form the heterocyclic rings. Accordingly, H.sub.2 O is a reaction product in either case and creates inherent difficulties in producing void-free composites. Voids have a deleterious effect on the shear strength, flexural strength, and dielectric properties of polyimide based composites.
In order to achieve high performance it has been previously proposed to use fully prereacted thermoplastic polyimides as the composite matrix. However, in this case the softening point or T.sub.g of the polyimide resin must be substantially above the intended use-temperature. Accordingly, a very high processing temperature is required which has the risk of causing pyrolytic degradation of the resin. Moreover, the pressure needed to achieve the required resin flow often is higher than commercially available equipment is capable of sustaining.
More recently, soluble polyimides, such as those described in Bateman et al U.S. Pat. No. 3,856,752 which is assigned to the same assignee as the present invention, have been described. Such polyimides require a volatile vehicle to achieve the required resin flow. As the vehicle is removed, however, the resin increases in viscosity. Accordingly, providing enough resin to achieve void free composites can be a problem.
Polyimide materials that are derived from in situ reacted monomers and oligomers have been used successfully in high performance environments. Processing problems normally associated with resin flow are less severe for such materials owing to their low molecular weight. One such material is PMR-15 which is described in U.S. Pat. No. 3,745,149. The acronym PMR stands for in situ polymerization of monomeric reactants. The -15 refers to a formulated molecular weight of 1500. PMR-15 is an addition polyimide derived from the dimethyl ester of benzophenone tetracarboxylic dianhydride (BTDE), the monomethyl ester of nadic anhydride (NE) and 4,4'-methylene dianiline (MDA). Addition polymerization is made possible by the use of the nadic end groups, which react without further evolution of volatiles at 250.degree.-350.degree. C.
While PMR-15 provides significant benefits, this resin and intermediate materials (e.g., prepregs) derived from it have certain disadvantages. Among these are toxicity, short shelf life, and handling difficulties during processing. The toxicity originates from MDA which is considered a suspect human carcinogen by the U.S. Environmental Protection Agency. Such real or perceived risks associated with PMR-15 are expected to hamper subsequent applications for this material.
Accordingly, there continues to be a need for addition-type polyimides for high performance applications, such as aerospace needs, which require elevated temperature performance in combination with reduced toxicity, chemical stability and greater ease in processing.